1. Field of the Invention
The invention relates generally to the field of petrophysical evaluation of subsurface rock formations. More specifically, the invention relates to methods and apparatus for quantitative determination of fluid saturation in pore spaces of rock formations based other than on electrical resistivity measurements of the formations.
2. Background Art
Porous subsurface rock formations are penetrated by wellbores for the purpose of extracting fluids from the pore spaces of such formations. In particular, oil and gas are extracted using such wellbores. It is important for economic reasons to determine what fractional volume of the pore spaces of penetrated formations are occupied by oil and/or gas prior to completion of construction of a wellbore. Methods known in the art for determining fractional volume of pore space occupied by connate water and by oil and/or gas are principally based on measurements of the electrical resistivity of the rock formations. In evaluating subsurface formations, a determined quantity is the fractional volume of pore space occupied by water (called “water saturation” and represented by Sw), wherein it is assumed that the non-water occupied pore space contains oil and/or gas.
Most techniques for determining water saturation from measurements of rock formation resistivity are based on research performed in the 1940s. See, Archie, G. E., Electrical Resistivity Log as an Aid in Determining Some Reservoir Characteristics, AIME Trans. 146, 1942, p. 54-62.
A relationship between the resistivity Rt of the porous rock to the connate water resistivity Rw, water saturation Sw and fractional volume of rock occupied by pore space (“porosity”) φ determined by Archie (see above) is
      R    t    =            R      w                      S        w        n            ⁢              ϕ        m            
The foregoing relationship has proven to be accurate for many formations that constitute hydrocarbon reservoirs. For water-wet rock formations, the exponents n and m of the Archie relationship above are generally both close to 2. The stability of these exponents in the case of water-wet formations has enabled making reasonably accurate reserves estimates for new reservoirs directly from resistivity and porosity measurements made from within wellbores (“well logs”). For sandstone rocks containing clay modified formulas are known to correct for the conductivity of clay.
It has been observed, in particular in limestone/dolomite rock formations (collectively “carbonates”), that the value of the foregoing exponents can vary quite significantly with respect to depth even within the same geologic rock formation. This is due to natural heterogeneities in carbonates, e.g., rock mineral composition and rock pore structure changes, wettability changes, etc.
Furthermore, if the formation being evaluated has been invaded with, for example, the liquid phase of fluid (“drilling mud”) used to drill the wellbore, the connate water originally in place in the pore spaces becomes mixed with varying amounts of the drilling fluid liquid (“mud filtrate”), and the salinity of the water in the pore spaces becomes difficult or impossible to determine using only resistivity measurements. Archie-formula derived water saturation values are therefore considered to be unreliable in many carbonate formations.
Other methods known in the art for determining water saturation include measurement of formation dielectric constant, nuclear magnetic resonance relaxation times and distributions thereof, neutron capture cross section and carbon/oxygen ratio. The foregoing measurements are generally limited in lateral depth into the formation from the wellbore wall of a few inches. As a result, at the time such instruments are inserted into the wellbore for measurements (typically after withdrawal of drilling tools and insertion therein on an armored electrical cable) the zones of measurement of the foregoing are generally completely invaded by mud filtrate, and the water saturation measurement does not reflect the oil and/or gas contained in the uninvaded rock.
Among the foregoing measurements the laterally deepest is neutron capture cross section. Such measurement is the most likely of the foregoing not be affected by mud filtrate invasion if the measurement is made while drilling of the wellbore. An instrument known by the trademark ECOSCOPE 6, which is a mark of the assignee of the present invention, is coupled within a drill string and provides neutron capture cross section measurements while drilling a wellbore. In many cases mud filtrate invasion is limited during drilling to less than the lateral depth of investigation of the various sensors on the ECOSCOPE instrument, which can thus provide measurements related to the concentration of chlorine in the rock formation. If the connate formation water salinity is known, the neutron capture cross section measurements can be directly used to determine Sw. However, the foregoing is not applicable to rock formations having low salinity (i.e. less than about 50,000 parts per million [“kppm”] sodium chloride concentration) connate water, and the foregoing instrument does not solve the challenge of invaded zones having unknown water salinity.
There continues to be a need for well logging techniques that can quantitatively determine water saturation in formations where the applicability of the Archie relationship is limited.